Field of the Invention
This invention generally relates to a heat exchanger adapted to be used in a vapor compression system. More specifically, this invention relates to a heat exchanger including a tube bundle with a falling film region, an accumulating region and a flooded region.
Background Information
Vapor compression refrigeration has been the most commonly used method for air-conditioning of large buildings or the like. Conventional vapor compression refrigeration systems are typically provided with an evaporator, which is a heat exchanger that allows the refrigerant to evaporate from liquid to vapor while absorbing heat from liquid to be cooled passing through the evaporator. One type of evaporator includes a tube bundle having a plurality of horizontally extending heat transfer tubes through which the liquid to be cooled is circulated, and the tube bundle is housed inside a cylindrical shell. There are several known methods for evaporating the refrigerant in this type of evaporator. In a flooded evaporator, the shell is filled with liquid refrigerant and the heat transfer tubes are immersed in a pool of the liquid refrigerant so that the liquid refrigerant boils and/or evaporates as vapor. In a falling film evaporator, liquid refrigerant is deposited onto exterior surfaces of the heat transfer tubes from above so that a layer or a thin film of the liquid refrigerant is formed along the exterior surfaces of the heat transfer tubes. Heat from walls of the heat transfer tubes is transferred via convection and/or conduction through the liquid film to the vapor-liquid interface where part of the liquid refrigerant evaporates, and thus, heat is removed from the water flowing inside of the heat transfer tubes. The liquid refrigerant that does not evaporate falls vertically from the heat transfer tube at an upper position toward the heat transfer tube at a lower position by force of gravity. There is also a hybrid falling film evaporator, in which the liquid refrigerant is deposited on the exterior surfaces of some of the heat transfer tubes in the tube bundle and the other heat transfer tubes in the tube bundle are immersed in the liquid refrigerant that has been collected at the bottom portion of the shell.
Although the flooded evaporators exhibit high heat transfer performance, the flooded evaporators require a considerable amount of refrigerant because the heat transfer tubes are immersed in a pool of the liquid refrigerant. With recent development of new and high-cost refrigerant having a much lower global warming potential (such as R1234ze or R1234yf), it is desirable to reduce the refrigerant charge in the evaporator. The main advantage of the falling film evaporators is that the refrigerant charge can be reduced while ensuring good heat transfer performance. Therefore, the falling film evaporators have a significant potential to replace the flooded evaporators in large refrigeration systems.
U.S. Pat. No. 5,839,294 discloses a hybrid falling film evaporator that has a section that operates in a flooded mode and a section that operates in a falling film mode. More specifically, the evaporator disclosed in this publication includes an outer shell through which passes a plurality of horizontal heat transfer tubes in a tube bundle. A distribution system is provided in overlying relationship with the upper most level of the heat transfer tubes in the tube bundle so that refrigerant which enters into the shell is dispensed onto the top of the tubes. The liquid refrigerant forms a film along an exterior wall of each of the heat transfer tubes where part of the liquid refrigerant evaporates as the vapor refrigerant. The rest of the liquid refrigerant collects in the lower portion of the shell. In steady state operation, the level of liquid refrigerant within the outer shell is maintained at a level such that at least twenty-five percent of the horizontal heat transfer tubes near the lower end of the shell are immersed in liquid refrigerant. Therefore, in this publication, the evaporator operates with the heat transfer tubes in the lower section of the shell operating in a flooded heat transfer mode, while the heat transfer tubes which are not immersed in liquid refrigerant operate in a falling film heat transfer mode.
U.S. Pat. No. 7,849,710 discloses a falling film evaporator in which liquid refrigerant collected in a lower portion of an evaporator shell is recirculated. More specifically, the evaporator disclosed in this publication includes the shell having a tube bundle with a plurality of heat transfer tubes extending substantially horizontally in the shell. Liquid refrigerant that enters in the shell is directed from a distributor to the heat transfer tubes. The liquid refrigerant creates a film along an exterior wall of each of the heat transfer tubes where part of the liquid refrigerant evaporates as the vapor refrigerant. The rest of the liquid refrigerant collects in a lower portion of the shell. In this publication, a pump or an ejector is provided to draw the liquid refrigerant collected in the lower portion of the shell to recirculate the liquid refrigerant from the lower portion of the shell to the distributor.